This invention relates to dental apparatus, and more particularly to dental post and core assemblies for use in the formation of a dental core as part of the restoration of a tooth structure, and specifically to devices commonly known as post and core systems as well as impression transfer posts.
In the restoration of damaged teeth, one technique that is frequently used is to prepare the tooth understructure or tooth root, and build upon it a superstructure including a core anchored with a post and upon it a crown. The technique generally includes preparing the tooth root through various procedures including endodontic treatment. The canal is then predrilled to form an enlarged bore coaxial with the root canal. A post is inserted into the pre-drilled bore and a core is fabricated and attached to it creating a retentive foundation. A crown is then fabricated as the final part of the restoration.
Various techniques have been established for forming the post and core. One method is referred to as the direct technique a procedure whereby the dentist at the chairside performs the necessary steps in sequence to prepare a pattern for casting the object. This involves the use of a plastic post which is inserted into the predrilled bore in the tooth root. Using appropriate wax or resin, a core is fabricated directly on the tooth structure. The core with the connected post is then sprued and invested in a conventional manner, and is cast using a typical "lost wax process". The cast post and core inserted and cemented in the dentition, becomes the foundation for the restoration of choice.
Using an indirect method, where the dentist preparing the tooth in every detail makes an impression of the dentition and the dental technologist performs the necessary tasks, in sequence, on a dental cast obtained from the impression and delivers to the dentist the final product, an impression is formed of the tooth root and surrounding area. The impression is then sent to a laboratory where technicians using standard procedures initially prepare a dental model from the impression. On the dental model, a core pattern is fabricated by conventional wax method and/or using self curing resin Appropriate posts are inserted into the dental model as part of the fabricated core. The fabricated core and post is sprued and invested and used to form the final cast core. This is inserted into the actual dentition and the final restoration of choice is formed.
In many cases, especially where the tooth is rather large, in addition to the central post, retention pins are utilized. Such pins provide additional securement of the core onto the tooth root. The use of the pins also prevents rotation of the core with respect to the tooth root.
One problem that is faced using these various known techniques is that the particular shape of the canal, and especially the mouth of the canal, is not regularly accommodated by the single central bore. Typically, the upper end of the canal is outwardly flared and often represents an oval or elliptical shape in cross section. The placement of the single cylindrical post to fit in the rather large mouth of the canal would not be feasible since the lower portion of the canal and the root are much narrower. Thus utilizing a larger cylindrical post would cause the weakening or side perforation of the root.
The problem of the wider mouth area of the canal structure has been addressed in U.S. Pat. No. 4,600,392, issued to the same inventor and assignee of the present invention. In that patent, there is described a dental post which is contoured to have ribs at the side of a central rod. In that patent, the post is utilized in a technique whereby the post is directly cemented in the bore and the core is mechanically secured to the post by a locking arrangement. The bore is initially formed as a central bore and subsequently, using jigs, ancillary bores on either side are formed in communication with the central bore but accommodating the side ribs on the post. One of the ribs is shorter than the other to distribute forces and to add resistance to them in a most advantageous manner.
While such post with side ribs has been used to directly connect a core, the problem of the wide mouth at the upper end of the canal has not been addressed in connection with casting of post and core systems.
An additional problem with both the direct and indirect methods is that when the initial central bore is formed, the depth of the bore is not accurately known. As a result, a post that is inserted into the central bore may not seat properly to the bottom of the bore.
Accordingly, there is need of an improved type of dental post assembly for use in connection with both the direct and the indirect techniques for forming core and post castings.